How can I add a thesaurus to the iOS "Look Up" menu

I think the "Define" menu used to have both dictionary and thesaurus capabilities, but it seems since it was changed to "Look Up", it got web and iTunes search, kept the dictionary but lost the thesaurus. I've tried adding dictionaries, but even with the American Oxford, British Oxford and Apple Dictionary each checked, I can't seem to get a thesaurus in the Look Up menu. Any ideas? I need a thesaurus more than definitions and web lookups.

Thanks